1. Field of the Invention
The present invention relates to a compressor using a motor as a driving, part, in which a rotational speed control is performed by means of, for example, an inverter, and to its control device and its control method.
2. Description of the Related Art
Conventionally, a method for operating a screw compressor that is adapted to perform a rotational speed control of a motor, which is a driving part of a lubricating type screw compressor body, by means of an inverter has been well known, as disclosed in Japanese Laid-Open Patent Publication Hei 6-10876. The screw compressor comprises a capacity control system including a P1 control unit, a pressure regulation valve and the inverter, besides an intake throttle valve in the inlet side of the lubricating type screw compressor body and a pressure sensor provided at the discharge side of the lubricating type screw compressor body so that it can detect a pressure.
According to the above operating method, a capacity control that maintains the pressure by means of the rotation speed control is performed in the case where a pressure detected by the pressure sensor rises, the rotational speed of the motor decreases through the PI control unit and the inverter, whereas the pressure drops, the rotational speed of the motor increases. However, the rotational speed control using the inverter and P1 control unit is discontinued just before the inverter is tripped by an overload in the course of decreasing the rotational speed, and then the rotational speed control is switched to a capacity control for maintaining the pressure to be constant by means of intake regulation: the capacity control opens the pressure regulation valve, and decreases the opening of the intake throttle valve when the pressure rises whereas increases the opening when the pressure drops. By such an operating method, energy saving and the like have been attempted.
Further, another method for operating a screw compressor has also been adopted conventionally that performs a capacity control by means of a loading/unloading control operating, wherein an intake regulation valve and a gas release valve are provided at the inlet side and discharge side of a screw compressor body, respectively, and the capacity control closes the intake regulation valve and opens the gas release valve when the pressure in the discharge side rises, whereas opens the intake regulation valve and closes the gas release valve when the pressure in the discharge side drops.
According to the method for operating the screw compressor disclosed in the Japanese Patent Laid-Open Hei 6-10876, the rotational speed of a motor is adapted to be decreased to the time just before the inverter is tripped if the pressure in the discharge side raises. Accordingly, the rotational speed is sometimes excessively decreased, so that a problem is caused in that the leakage quantity of gas, compressed within the screw compressor body, into the inlet side may be increased and the compression efficiency may be decreased. The increase of leakage quantity is especially eminent in a non-lubricating type screw compressor with gaps between screw rotors and between screw rotors and walls of rotor chambers being not oil-sealed, rather than in a lubricating type screw compressor with the gaps being oil-sealed. In addition, there is also a problem in that abnormal temperature rising is caused in the discharge side as the leakage quantity is increased.
Meanwhile, in the method for operating the screw compressor that performs the loading/unloading control operating, there is a problem in that its mechanical loss is large and its efficiency is poor in view of power consumption.
The present invention is conceived to eliminate such problems of the prior art and it is an object of the present invention to provide an apparatus for controlling a compressor that enables the increase of compression efficiency, the prevention of abnormal temperature rising in the discharge side, and the reduction of power loss.
In order to solve the above problem, the present invention is a compressor comprising: a compressor body that is driven by a motor; a motor control mechanism for controlling the rotational speed of said motor; a pressure detector for detecting a discharge pressure of said compressor body; an intake regulation valve provided at the inlet side of said compressor body; and a control operation unit that outputs a motor rotational speed command signal to said motor control mechanism and outputs an open/close command signal to said intake air regulating valve, wherein when the rotational speed commanded to said motor control mechanism is not lower than a predetermined threshold, said control operation unit outputs the motor rotational speed command signal to said motor control mechanism, as it outputted the open signal to said intake regulation valve, so that said discharge pressure will be maintained at a predetermined target pressure, based on the pressure signal from said pressure detector, and when the rotational speed commanded to said motor control mechanism is lower than said threshold, said control operation unit outputs the open/close signal to said intake regulation valve in a state that the rotational speed of said motor is being maintained at about said threshold, so that said discharge pressure will be maintained at the predetermined target pressure, based on the pressure signal from said pressure detector. Here, said threshold is set to such a value that the leakage quantity of gas compressed by said screw compressor, into the inlet side of said screw compressor will not be increased as the rotational speed of said motor is decreased.
In other word, said threshold is set to a value which is sufficiently larger than a rotational speed at which said motor trips.
As to the relationship with the rated rotational speed of motor, it is quite preferable for said threshold to be set 30% or more of the rated rotational speed of said motor.
According to the above construction, it is possible to realize the increase of compression efficiency, the prevention of abnormal temperature rising in the discharge side, and the reduction of power loss, because the rotational speed of said motor is not excessively decreased in performing a control such as controlling the pressure fluctuation in the discharge side.
In addition, it is possible that said motor control mechanism comprises an inverter.
Moreover, it is quite preferable if said control operation unit performs a proportional integral derivative (PID) control operation based on the deviation from said target pressure of said pressure signal and also has an integration limit in performing the integration calculation, when said control operation unit calculates the motor rotational speed command signal to said motor control mechanism in order to maintain said discharge pressure at the predetermined target pressure based on the pressure signal from said pressure detector.
The present invention is also an apparatus for controlling a compressor that comprises a compressor body driven by a motor and a motor control mechanism for controlling a rotational speed of said motor, the apparatus comprising: a pressure detector for detecting a discharge pressure of said compressor body; an intake regulation valve provided at the inlet side of said compressor body; and a control operation unit that outputs a motor rotational speed command signal to said motor control mechanism and outputs an open/close command signal to said intake air regulating valve, wherein when the rotational speed commanded to said motor control mechanism is not lower than a predetermined threshold, said control operation unit outputs the motor rotational speed command signal to said motor control mechanism, as it outputted the open signal to said intake regulation valve, so that said discharge pressure will be maintained at the predetermined target pressure, based on the signal pressure from said pressure detector, and when the rotational speed commanded to said motor control mechanism is lower than said threshold, said control operation unit outputs the open/close signal to said intake regulation valve, in a state that the rotational speed of said motor is being maintained at about said threshold, so that said discharge pressure in the predetermined target pressure will be maintained at said predetermined threshold, based on the pressure signal from said pressure detector. Here, said threshold is set to such a value that the leakage quantity of gas compressed by said screw compressor into the inlet side of said screw compressor will not be increased as the rotational speed of said motor is decreased.
The present invention is a method for controlling a compressor that comprises a compressor body driven by a motor and a motor control mechanism for controlling a rotational speed of said motor, and is provided with an intake regulation valve in the inlet side thereof, the method comprising steps of: detecting a discharge pressure of said compressor body by means of a pressure detector; comparing the rotational speed currently commanded to said motor control mechanism with a predetermined threshold; and when said commanded rotational speed is not lower than the predetermined threshold, controlling the rotational speed of said motor so that said discharge pressure will be maintained at a predetermined target pressure based on the pressure signal from said pressure detector, as an open signal was outputted to said intake regulation valve, and when said commanded rotational speed is lower than said threshold, performing an open/close control using said intake regulation valve so that said discharge pressure will be maintained at the predetermined target pressure based on the pressure signal from said pressure detector, in a state that the rotational speed of said motor is being maintained at about said threshold. Here, said threshold is set to such a value that the leakage quantity of gas, compressed by said screw compressor, into the inlet side of said screw compressor will not be increased as the rotational speed of the motor is decreased.